一曲线的方程为 y=e2xx−2y = \frac{e^{2x}}{x-2},其中 x>2x > 2。

  1. (a)

    求 dydx\dfrac{dy}{dx}。

  2. (b)

    求驻点坐标的精确值。

  3. (c)

    确定驻点的性质。

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完整解法

3(a)

1. Apply the quotient rule to differentiate the function

Let u=e2xu = e^{2x} and v=x−2v = x-2.
Then dudx=2e2x\dfrac{du}{dx} = 2e^{2x} and dvdx=1\dfrac{dv}{dx} = 1.
Using the quotient rule dydx=vdudx−udvdxv2\dfrac{dy}{dx} = \dfrac{v \frac{du}{dx} - u \frac{dv}{dx}}{v^2}:
dydx=(x−2)(2e2x)−(e2x)(1)(x−2)2\frac{dy}{dx} = \frac{(x-2)(2e^{2x}) - (e^{2x})(1)}{(x-2)^2}

2. Simplify the derivative

dydx=e2x[2(x−2)−1](x−2)2\frac{dy}{dx} = \frac{e^{2x}[2(x-2) - 1]}{(x-2)^2}
=e2x(2x−4−1)(x−2)2= \frac{e^{2x}(2x - 4 - 1)}{(x-2)^2}
=e2x(2x−5)(x−2)2= \frac{e^{2x}(2x - 5)}{(x-2)^2}

答案

e2x(2x−5)(x−2)2\dfrac{e^{2x}(2x - 5)}{(x-2)^2}

3(b)

1. Set the derivative from part (a) to zero to find the stationary point

From part (a), dydx=e2x(2x−5)(x−2)2\frac{dy}{dx} = \frac{e^{2x}(2x - 5)}{(x-2)^2}.
For a stationary point, dydx=0\frac{dy}{dx} = 0.
e2x(2x−5)(x−2)2=0\frac{e^{2x}(2x - 5)}{(x-2)^2} = 0

2. Solve for the $x$-coordinate of the stationary point

Since e2x>0e^{2x} > 0 for all xx, we have:
2x−5=02x - 5 = 0
x=52x = \frac{5}{2}

3. Find the corresponding exact $y$-coordinate

Substitute x=52x = \frac{5}{2} into the original equation:
y=e2(52)52−2y = \frac{e^{2(\frac{5}{2})}}{\frac{5}{2} - 2}
y=e512y = \frac{e^5}{\frac{1}{2}}
y=2e5y = 2e^5
The coordinates are (52,2e5)(\frac{5}{2}, 2e^5).

答案

(52,2e5)(\frac{5}{2}, 2e^5)

3(c)

1. Use the first derivative test to check the sign of the gradient around the stationary point $x = 2.5$.

From earlier parts, dydx=e2x(2x−5)(x−2)2\frac{dy}{dx} = \frac{e^{2x}(2x - 5)}{(x-2)^2} and the stationary point is at x=2.5x = 2.5.

For xx slightly less than 2.52.5 (e.g., x=2.4x = 2.4):
2x−5<02x - 5 < 0, e2x>0e^{2x} > 0, and (x−2)2>0(x-2)^2 > 0, so dydx<0\frac{dy}{dx} < 0.

For xx slightly greater than 2.52.5 (e.g., x=2.6x = 2.6):
2x−5>02x - 5 > 0, e2x>0e^{2x} > 0, and (x−2)2>0(x-2)^2 > 0, so dydx>0\frac{dy}{dx} > 0.

2. Conclude the nature of the stationary point.

Since the gradient changes from negative to positive as xx increases through 2.52.5, the stationary point is a minimum point.

答案

Minimum point

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